Post
deadline Session on MgB2
Talks 60 - 79
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Slideshows containing audio and
slides are available for all linked talk titles. Not all talks are linked.
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Table of Contents
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| Talks: 1-19 20-39 40-59 60-79 | |
| Talk 60 | Magnetic Transitions in MgB2 |
| Talk 61 | High Temporal Stability of Supercurrents in MgB2 |
| Talk 62 | Current transport and magnetic relaxation near and below the irreversibility |
| Talk 63 | Critical Currents and Flux Creep in MgB2 |
| Talk 64 | Magneto-optical study of flux penetration in MgB2 |
| Talk 65 | Temperature dependence of the intergranular critical currents in MnB2 ceramic |
| Talk 66 | Giant quantum fluctuation of vortices in the new superconductor MgB2 |
| Talk 67 | The new superconductors. |
| Talk 68 | The upper critical field problem in MgB2 |
| Talk 69 | Ab-initio calculations of lattice dynamics and electron-phonon coupling in MgB2 |
| Talk 70 | Measurement of the B |
| Talk 71 | dHvA measurements in MgB2 |
| Talk 72 | Soft X-ray Spectroscopy measurements of MgB2 at the B-K edge |
| Talk 73 | Investigation of Electronic Excitations in MgB2 by Raman Spectroscopy |
| Talk 74 | High-Frequency Studies of MgB2 |
| Talk 75 | Pressure Effects of MgB2 Superconductors on Its Tc and Crystal Lattice |
| Talk 76 | Carbon substitution effect in MgB2 |
| Talk 77 | Improved superconducting thin films of MgB2 on Si |
| Talk 78 | Superconducting MgB2 thin films by pulsed laser deposition. |
| Talk 79 | Te Amplification by Shape Resonance |
L.D. Cooley, S. Patnaik, D.C. Larbalestier,
A. Gurevich, X.Y. Cai, A.A. Polyanskii, M. Rikel, J.Y. Jiang, X. Song,
S. E. Babcock, C.B. Eom, and E.E. Hellstrom, Applied Superconductivity Center, University of Wisconsin–Madison.
Magnetization data from 0-14 T and 4.2-45
K were acquired for several MgB2 samples obtained from different
sources. The derived values
of the upper critical field Hc2
and the field HK
at which Kramer plots extrapolate to zero flux pinning show little variation,
despite the range of sample homogeneity and the differences in preparation. Since some samples were reacted with an excess
of Mg while others were made in an excess of B, these data suggest there
is very little variation of the stoichiometry of the primary superconducting
regions. The variation of HK(T) is linear and proportional
to Hc2(T), which
is not the expected behavior for an irreversibility line. A tail is also visible on all Kramer plots
above HK, indicating
that residual irreversible current loops persist to near Hc2.
High
Temporal Stability of Supercurrents in MgB2
J.R. THOMPSON, ORNL/University of Tennessee,
M. PARANTHAMAN, D.K. CHRISTEN, Oak Ridge National Lab, K.D. SORGE, H.J.
KIM, University of Tennessee, J.G. OSSANDON, University of Talca, Chile
Fine grained polycrystalline samples of
MgB2 superconductor were synthesized by reaction of the pure
elements at 890 ˚C for 2 h in a sealed Ta tube within an evacuated
quartz ampule. Sintered or powdered materials, containing <5 % of
impurity phases per X-ray powder diffractometry, had a sharp transition
with a midpoint Tc = 38.5 K. The supercurrent density J(T,H,t)
in the vortex state was studied as function of temperature T, magnetic
field H, and time t. The current J is highly stable in time,e.g., the “flux creep” rate S = -dln(J)/dln(t) ≤ 0.02
in fields of 1 Tesla, for temperatures up to Tc/2. Compared
with high-Tc’s, the decay rate S is smaller by a factor of
3-10 or more. In addition, the persistent current density J decreases
approximately linearly with T, in contrast to the quasi-exponential
falloff in HTSC’s; this is another manifestation of the highly stable
J(t).
Current
transport and magnetic relaxation near and below the irreversibility
line of MgB2
X.Y. Cai, A. Gurevich, S. Patnaik, A.A.
Polyanskii, M. Rikel, J.Y. Jiang, L.D. Cooley, C.B. Eom, E.E. Hellstrom,
and D.C. Larbalestier, Applied Superconductivity Center, University
of Wisconsin–Madison, S.L. Bud’ko, C. Petrovic, G. Lapertot, C.E.
Cunningham, and P.C. Canfield, Ames
National Laboratory.
We performed magnetic flux creep and transport
measurements on sintered MgB2 samples in magnetic fields
near and below the irreversibility line B* » 0.2(Tc – T) [Tesla]. The dimensionless
flux creep rate s(T,B) = dlnM/dlnt was found to decrease with the magnetic
field B and temperature T, from s ~ 10-2 at B << B* to s ~ 1 at B » B*. Resistive measurements near B* enabled
us to trace the crossover from a nonlinear current-voltage characteristic
below B* to the flux flow behavior above B*.
Critical
Currents and Flux Creep in MgB2 (no slides, audio only)
Garry Perkins, Yura Bugoslavsky,
Max Polichetti, Xiaoding Qi, Alexei Zhukov, Lesley Cohen, Judith Driscoll.
Centre for High Temperature Superconductivity, Imperial College, London
SW7 2BZ, UK
We report detailed measurements
on small (~ 100 micron) polycrystalline fragments of MgB2
of critical current densities Jc and flux creep rate S. The natural grain boundaries within the fragments
appear fully transparent to supercurrent. Within the framework of collective
pinning theory that has been applied to HTS materials, the dependences
of Jc and S on field suggest a catastrophic decline in pinning
energy of pure MgB2 with increasing field.
Magneto-optical
study of flux penetration in MgB2
A.A. Polyanskii, J.Y. Jiang, M. Rikel, S.
Patnaik, X.Y. Cai, L.D. Cooley, A. Gurevich, C.B. Eom, E.E. Hellstrom,
and D.C. Larbalestier, Applied Superconductivity Center, University of Wisconsin–Madison, S.L.
Bud’ko, C. Petrovic, G. Lapertot, C.E. Cunningham, and P.C. Canfield,
Ames National Laboratory.
Magneto-optical (MO) studies of flux penetration
in sintered MgB2 samples and MgB2 wires were performed.
The sintered samples showed well developed roof patterns, rather uniform
critical state and Bean-like flux profiles, which enabled us to extract
bulk magnetization current density Jc(T) in the temperature
range 12K < T < Tc and magnetic fields, H < 1200
Oe. The wires showed much more nonuniform flux
penetration due to material inhomogeneities and microcracks. From the
MO flux profiles we obtained the local Jc values of order
105 A/cm2, in agreement with magnetization measurements.
Temperature
dependence of the intergranular critical currents in MnB2 ceramic
samples.
V.K.Vlasko-Vlasov, U.Welp, G.W.Crabtree,
D.J.Miller, D.Hinks Argonne
National Laboratory, Argonne, IL
Magneto-optical measurements of the flux
distributions in MnB2 samples reveal regions of high critical
currents connected by noticeably weaker current links. This is in accordance
with polarized optics and electron microscopy pictures showing dense
and less dense compacts of small single crystal MnB2 grains.
From the average slopes of the trapped flux profiles the weak intergranular
currents, limiting the current carrying ability of the samples, and
their temperature dependence were determined. Currents in the well coupled
regions are estimated to be an order of magnitude larger.
Giant
quantum fluctuation of vortices in the new superconductor MgB2
H. H. Wen*, S. L. Li, Z. W. Zhao, Y. M.
Ni, Z. A. Ren, G. C. Che, H. P. Yang, Z. Y. Liu and Z. X. Zhao. National
Lab for Superconductivity, Institute of Physics and Chinese Academy
of Sciences, P. O. Box 603, Beijing 100080, P. R. China
The recently discovered new superconductor
MgB2 generates enormous interests in the field of superconductivity.
One big issue concerns in which region on the field-temperature phase
diagram it can carry large critical current density ( jc
) and thus can be used in the future for industry. This jc
is controlled by the mobility of the magnetic vortex tubes ( vortices
) and vanishes at the melting line between the vortex solid and liquid.
This melting can be induced by strong fluctuation of the vortex position
by either thermal or quantum effect. At T = 0 K only the quantum fluctuation
is left. In this report we present the evidence for a vortex quantum
liquid induced by giant quantum fluctuation effect at zero K. Further
investigation on the magnetic relaxation at different fields may indicate
that the melting at a finite temperature is also governed by this effect
posing a strong limit to the application of this new superconductor
in a high field.
The
new superconductors. (no slides, audio only)
John D. Dow, Arizona State University
Two new potential superconductors have raised
considerable interest recently: MgB(2) (T(C)=39 K) [1] and Ag(x)Pb(6)CO(9)
(T(C)=342 K) [2]. Both materials
appear to have hole carriers. MgB(2)
appears to be a “low-temperature” phonon superconductor (with a rather
high temperature), and Ag(x)Pb(6)CO(9) may be a genuine “high-temperature”
superconductor, if its superconductivity is confirmed.
[1] J. Nagamatsu, N. Nakagawa, T. Muranaka,
Y. Zenitani, and J. Akimitsu,
“Superconductivity at 39 K in magnesium
diboride,” to be published.
[2] D. Djurek, Z. Meduniae, A. Tonejc, and
M. Paljeviae, “Onset of superconductivity at 342 K in a novel chain
conductor Ag(x)Pb(6)CO(9) (0.7<x<1),” to be published.
The
upper critical field problem in MgB2
H. ROSNER, W.E. PICKETT, Department of Physics, UC Davis, S.
SHULGA, S. -L. DRECHSLER, H. ESCHRIG, Institute for Solid State and
Material Research Dresden, Germany.
The upper critical field HC2(T) for MgB2
is analyzed in terms of single and multi-band Eliashberg models. The
relatively high value of HC2(0) ≈ 14 to 18 T can be
understood, if a strongly coupled subgroup of heavy quasiparticles is
involved in the superconductivity. The sizable coupling to both low-
and high-frequency bosons is essential. This picture is supported by
LDA calculations of Fermi velocity distribution over different sheets
of the Fermi surface with special emphasis on the hole tubes. The possible
origin of anharmonic soft modes is briefly discussed. Similarities and
differences with transition metal borocarbides are figured out.
Ab-initio calculations of lattice dynamics and electron-phonon coupling in MgB2
V.
OZOLINS and M. VAN SCHILFGAARDE, Sandia National Laboratories, Livermore,
California.
We
have carried out first-principles linear response calculations of phonon
densities of states and electron-phonon interaction in MgB2.
Complete phonon densities of states, electron-phonon spectral function
a2F(w) and l for MgB2 will be presented. The nature of the lattice
dynamics and its relation to the strong electron-phonon coupling in
this system will be discussed.
CORMAC
McGUINNESS and KEVIN E. SMITH,
Department of Physics, Boston University, 590
Commonwealth Ave, Boston, MA 02215, J. GUO and J. NORDGREN, Department
of Physics, Uppsala University, Box 530, 75121 Uppsala, Sweden,
T. VOGT, J. REILLY, J. F. TU, G. SCHNEIDER and
P. D. JOHNSON, Department of Physics,
Brookhaven National Laboratory, Upton, NY 11973.
The valence
band electronic structure of MgB2 has been measured using
soft x-ray emission (SXE) and soft x-ray absorption (SXA) spectroscopies. SXE provides direct measurement of the occupied,
elementally resolved, partial density of states (PDOS) in solids.
SXE is bulk sensitive due to a sampling depth of approximately
1000 Å, and is insensitive to the degree of sample crystallinity.
The room temperature B 2p
valence band PDOS was obtained by recording the radiative de-excitation
of a B 1s core hole. The B 1s hole was created
with monochromatic soft x-ray radiation from an undulator device at
the Advanced Light Source. Photon
energies well above the excitation threshold were used in the PDOS measurement. The SXE measurements of the occupied PDOS agree
well with a tight binding calculation. Likewise, the unoccupied B 2p PDOS measured using SXA is similar to
the calculated PDOS with one exception: an intense narrow absorption
feature is observed just above the absorption threshold.
We speculate that this feature is excitonic in nature. Resonant
excitation at this absorption energy results in strong inelastic scattering
and a dramatic change in the observed x-ray emission spectrum.
E. C. Palm, S. W. Tozer, Donavan Hall, and T. P. Murphy, National High Magnetic Field Laboratory Florida
State University, Tallahassee, FL
We report de Haas - van Alphen (dHvA) measurements made on
isotopically pure Mg11B2 powder and dense MgB2
wires. The powder samples were mixed with epoxy and oriented in a 33
T field while the epoxy cured. The measurements were performed at temperatures
between 0.5 K and 40 K in magnetic fields up to 33 T using a high quality
torque cantilever magnetometer. Typical data, measured frequencies of
the oscillations and effective masses of principle orbits, will be presented.
Soft
X-ray Spectroscopy measurements of MgB2 at the B-K edge
T.A. CALLCOTT,
L. LIN, G.T. WOODS, J. R. THOMPSON,
Univ. of Tennessee,
We report
soft x-ray absorption and fluorescence measurements at the boron K-edge
of superconducting MgB2.
Three notable features are found.
The measurements confirm a high density of B p-states at the
Fermi edge and extending to approximately 0.8 eV above the edge,
which is derived from B-s (2pxy). orbitals They also show
a strong exciton feature indicating strong localization in the out-of-plane
p (2pz). orbiatls.
Finally, a second excitonic feature is observed that derives
from boron oxide contamination of the sample.
Investigation of Electronic Excitations in MgB2 by Raman Spectroscopy
X. K. CHEN, M. J. KONSTANTINOVIC, J. C. IRWIN, Department of Physics, Simon Fraser University, Burnaby, B.C., V5A 1S6,
Canada; J. P. FRANCK, Department
of Physics, University of Alberta, Edmonton, A.B., T6G 2J1, Canada.
The
Raman spectra of polycrystalline MgB2 have been measured
from 50 cm-1 to 1200 cm-1 at temperatures above
and below Tc, using parallel and crossed polarization scattering
geometries. A broad continuum due to scattering from electronic
excitations was found to behave similarly to the electronic continua
observed in the high-Tc cuprates.
When the temperature was decreased below Tc, a superconductivity-induced
redistribution in the continuum was observed. A pair-breaking peak at 110 cm-1
due to excitations across the superconducting gap appeared in both scattering
geometries. The peak frequency
and its polarization dependence indicate an isotropic s-wave gap with
2D/kBT»4.2, which corresponds to the strong-coupling
regime.
High-Frequency
Studies of MgB2
S. E. Lofland, Dept. of Physics and Chemistry, Rowan University,
Glassboro, NJ, Dept. of Physics, University of Maryland, College Park,
S. D. Tyagi,
Dept. of Physics, Drexel University, Philadelphia,
PA,
K. V. Ramanajuchary, M. Botsford,
Dept. of Physics
and Chemistry, Rowan University, Glassboro, NJ
We present microwave absorption and susceptibility studies of MgB2. We find that the absorption is well described
by the London model, yielding a reasonable for the penetration depth.
In addition, we find that the number of weak links is much smaller
than in HTSC; however, they are not completely negligible.
Frequency-dependent susceptibility studies show that the vortex
pinning is very strong.
Pressure
Effects of MgB2 Superconductors on Its Tc and
Crystal Lattice
J. Tang,1 A. Matsushita,1 Y. Takano,1
K. Togano,1 H. Kito,2 H. Ihara,2 L.-C.
Qin 3
"Times New Roman"'>1
National Research Institute for Metals,
1-2-1 Sengen, Tsukuba 305-0047, Japan
2 Electrotechnical Laboratory, 1-1-4
Umezono, Tsukuba 305-8568, Japan
3
JST-ICORP Nanotubulite Project, c/o
NEC Corp., Tsukuba 305-8501, Japan
Hydrostatic pressure up to 10 GPa has been applied to determine
the dependence of transition temperature Tc and lattice properties
of an MgB2 superconducting material that is prepared by high
pressure (3.5 GPa) and high temperature (1000 ºC). This material shows
narrowed region in temperature during the transition from normal state
to superconducting state. The transition temperature Tc decreases with
increasing pressure linearly at a rate of 1.03 K/GPa. The crystal lattice
properties, measured with a diamond anvil cell using in situ synchrotron
X-ray diffraction, show anisotropy with its c-axis being more compressible
than the a-axis.
Correspondence:
tang@nrim.go.jp
Carbon
substitution effect in MgB2
Jai Seok Ahn1 and Eun Jip Choi1,2,
1
Center for Strongly Correlated Materials Research, Seoul National University, Seoul 151-742, Korea,
2
Department of Physics, University of Seoul, Seoul 130-743, Korea
We investigated carbon substitution effect on boron plane of superconducting
MgB2. MgB2 and MgB1.8C0.2
samples are synthesized under high pressure furnace. MgB1.8C0.2
are characterized as AlB2-type single phase with smaller
B-B distance. During the superconducting transition, two distinct onset
temperatures are observed in MgB1.8C0.2.
Improved superconducting thin films of MgB2 on Si.
A.
BRINKMAN, D. MIJATOVIC, G. RIJNDERS, V. LECA, H.J.H. SMILDE, I. OOMEN,
A. A. GOLUBOV, F. ROESTHUIS, S. HARKEMA, H. HILGENKAMP, D.H.A. BLANK
and H. ROGALLA. Low Temperature
Division, MESA+ Research Institute and Faculty of Applied
Physics, University of Twente, Enschede, The Netherlands, and C.A.J.
DAMEN, TSST, Hengelo, The Netherlands.
In a follow-up
of our cond-mat/0103198, new results will be presented on improved superconducting
thin films prepared on Si-substrates, using pulsed laser deposition
from a target composed of a mixture of Mg and MgB2 powders.
The films are deposited at room temperature and subsequently in-situ post-annealed. More information
will be presented on the optimization of the background pressure of
Ar gas in the deposition-chamber, for which the color of the plasma
is used as indicator.
Superconducting
MgB2 thin films by pulsed laser deposition.
S.
R. SHINDE, S. B. OGALE, R. P. SHARMA, R. L. GREENE and T. VENKATESAN,
Center for Superconductivity Research, Department of Physics, University
of Maryland, College Park, MD 20742-4111, P. C. CANFIELD, S. L. BUD’KO,
G. LAPERTOT, Ames Laboratory, U. S. Department of Energy and Department
of Physics and Astronomy, Iowa State University, Ames, Iowa 50011
Thin
films of MgB2 exhibiting susceptibility and resistivity transitions
(TC) between 20-30 K are grown by in situ pulsed laser deposition
process. Different approaches involving ablation from a stoichiometric
target under different growth conditions, as well as multiplayer deposition
involving interposed Mg layers are examined and analyzed. Other strategies
such as post-deposition-annealing at 900 C with excess Mg (an ex situ
process) are also studied. Magnetic measurements on such samples show
TC as high as 38 K. These data will be presented and discussed.
Te Amplification by Shape Resonance
A. Bianconi*, D. Di Castro, S. Agrestini, G. Campi, N. L. Saini, Department of
Physics, and IBFM University of Roma "La Sapienza", P. le Aldo Moro 2, 00185
Roma, Italy.
A. Saccone, S. De Negri, M. Giovannini, Dipartimento di Chimica e Chimica
Industriale, Universite di Genova, Via Dodecaneso 31, 16146 Genova, Italy.
We have studied the variation of Tc as a function of charge density and lattice
parameters in Al doped MgB2 superconducting samples. We show that high Tc
superconductivity in MgB2 is realized by tuning the chemical potential near a
"shape resonance" as described in the patent for "high-temperature
superconductors made by metal heterostructures at the atomic limit". The
metallic MgB2 heterostucture at the atomic limit made by chemical reactions is
used for Tc amplification according with the claim of the patent at the n=2 "shape
resonance" for the superlattice of boron monolayers. For further information:
Presenting author, email:
http://www.superstripes.com
antonio.bianconi@roma1.infn.it